Modern automobiles equipped with automatic transmissions typically include a Park Lock feature and a Brake Transmission Shift Interlock (BTSI) feature. The operation of these features is controlled with respect to the position of a conventional ignition switch. With an ignition key inserted in the conventional ignition switch, the switch can be rotated to five different positions. These five positions are commonly referred to as the ACCESSORY, LOCK, OFF, RUN, and START positions. The ignition key can only be inserted into the ignition switch and removed from the ignition switch when the switch is disposed in the LOCK position. A counter-clockwise rotation of the ignition switch from the LOCK position moves the switch to the ACCESSORY position. A clockwise rotation of the ignition switch from the LOCK position moves the switch to the OFF, RUN, and START positions respectively.
Conventional automatic transmissions include a shifter or shift lever for selecting one of seven different operating positions. These operating positions are commonly referred to as the PARK, REVERSE, NEUTRAL, DRIVE, SECOND, and FIRST positions. An OVERDRIVE position is also provided for some transmissions.
The Park Lock feature is designed to prohibit movement of the shift lever from the PARK position unless the ignition switch is disposed in the RUN position. The Brake Transmission Shift Interlock feature is designed to prohibit movement of the shift lever from the PARK position unless the ignition switch is disposed in the RUN position and the brake pedal is depressed. With the ignition switch disposed in the RUN position, the BTSI feature is disabled when the brake pedal is sufficiently depressed to activate the brake light switch. When the BTSI feature is disabled, the shift lever may be removed from the PARK position.
As a result, a vehicle equipped with an automatic transmission cannot be placed in a condition to power the drive wheels in either a reverse or forward direction until the steering wheel has been freed for normal driving by release of the Park Lock feature and the driver has immobilized the vehicle by placing his or her foot on the brake pedal. In addition, the vehicle cannot be placed in NEUTRAL unless the key is in the ignition. This is a desired feature especially if the vehicle is parked on a slope as it could begin rolling down the slope if the vehicle is placed in NEUTRAL. It is generally believed that this combination of safety interlocks is effective in reducing accidental or inadvertent movement of a vehicle.